The execution of a zinc phosphate-based conversion treatment on various metals prior to the coating or plastic working thereof is known at present for the purpose of improving the paint adherence and post-paint corrosion resistance and improving the lubrication during plastic working.
The conversion treatment baths used for zinc phosphate-based conversion treatment are essentially acidic aqueous solutions that contain zinc ions, phosphate ions, and oxidizing agent(s). Nitrite salts, chlorate salts, hydrogen peroxide, organic nitro compounds, hydroxylamine, and the like, are ordinarily considered for this oxidizing agent. These oxidizing agents are typically called conversion "accelerators" because they function to accelerate the conversion reactions. Nitrate salts may be present in conversion baths, but--because, in the concentrations usually present in zinc phosphate-based conversion baths, nitrate ions do not exercise an oxidizing activity sufficient to convert ferrous ions substantially completely to ferric ions--nitrate ions must be distinguished from the conversion accelerators.
One important role of conversion accelerators during the zinc phosphate based conversion treatment of ferriferous metals is to oxidize the divalent iron ions eluting into the conversion bath to trivalent iron ions. For example, the conversion reactions are inhibited when divalent iron ions accumulate in the conversion bath during the continuous conversion treatment of ferriferous metals, and the role of the conversion accelerator in inhibiting this accumulation of divalent iron ions is thus crucial.
However, each of these already known conversion accelerators is associated with problems that must be addressed. For example, in the case of the nitrite salts, which are the most widely used conversion accelerators at the present time, these compounds are unstable in the acid region. As a result, these compounds undergo spontaneous decomposition and are thereby consumed even when conversion treatment is not being run (storage period). The maintenance of a constant or prescribed concentration of these compounds therefore requires continual replenishment to make up for the amount lost to this consumption.
It is also known that as a result of their oxidative activity and spontaneous decomposition these nitrite salts partially convert to NO.sub.x gas, which diffuses into and pollutes the atmosphere.
When chlorate salts are used as conversion accelerators, chloride ions are produced as a decomposition product during conversion treatment and accumulate in the conversion treatment bath. The corrosion resistance of the metal substrate is substantially impaired when even a trace of chloride ions from the conversion treatment bath remains on the surface of the metal workpiece. In addition, chlorate salts are ordinarily used in combination with another conversion accelerator, such as nitrite salts, and when used alone provide only a significantly reduced conversion reaction rate.
Stability in the conversion treatment bath is also a problem for the use of hydrogen peroxide as a conversion accelerator: Hydrogen peroxide is readily decomposed by oxygen dissolved in the conversion bath. In addition, hydrogen peroxide has a narrow optimal concentration range for conversion treatment, which makes it difficult to manage the conversion treatment bath. When the dissolved concentration is too high, a powdery, poorly adherent conversion coating is deposited on the metal surface.
With regard to the use of nitrogenous organic compounds as conversion accelerators, the following problems are associated with the use of organic nitro compounds such as nitroguanine and sodium m-nitrobenzenesulfonate: Nitroguanine, for example, has a low solubility in water and as a result cannot be formulated as a concentrate for addition to the conversion bath. It is also difficult to control the divalent iron ions concentration in the conversion bath using nitroguanine because this compound has a weak capacity to oxidize the divalent iron ions. On the other hand, sodium m-nitrobenzenesulfonate provides a poor conversion performance when used by itself, and for this reason this compound must ordinarily be used in combination with another, more powerful conversion accelerator. Moreover, its concentration management requires the use of large-scale measurement equipment, such as an ion chromatograph. Another problem with the use of organic nitro compounds is that the accumulation of these compounds and their decomposition products in the conversion bath causes an increase in the chemical oxygen demand ("COD") of the conversion treatment effluent, which unfavorably affects the environment.
Hydroxylamine compounds are another type of nitrogenous organic compounds used as conversion accelerators. These compounds, however, in order to achieve the best results, must be added to give concentrations of at least 1,000 parts per million by weight (hereinafter usually abbreviated as "ppm") in the conversion bath, giving rise to the possibility of a large and economically undesirable consumption of the conversion accelerator.
In addition, results have been reported from an investigation into the use of chromic acid and permanganate salts as conversion accelerators for zinc phosphate-based conversion treatment baths (Norio Sato, et al., Boshoku Gijutsu English title: Corrosion Engineering!, Volume 15, No. 5 (1966)).
These authors report that the formation of conversion coatings was not observed at concentrations of 5 millimoles per liter (hereinafter usually abbreviated as "mmol/L") or 10 mmol/L.
Many of the known conversion accelerators as described above are nitrogenous compounds, and as such resist removal by chemical wastewater treatment techniques, so that in practice they are usually removed through microbiological treatments. However, even with the use of microbiological treatments, the elimination of high concentrations of these nitrogenous compounds is highly problematic, while a complete elimination cannot be achieved even at low concentrations. Nitrogenous compounds have recently come to be thought of as one factor in the eutrophication of bodies of water, and the discharge of nitrogenous compounds has therefore become subject to an increasingly strict regulatory atmosphere. In view of these environmental considerations, the development of a nitrogenous compound-free zinc phosphate-based conversion bath would be highly desirable.
Another drawback to each of the above-described conversion accelerators is that, in order to obtain the thin, uniform, fine, and dense conversion coatings desired as underpaint coatings, the metal surface must in each case be conditioned by treatment with a colloidal titanium system immediately prior to execution of the conversion treatment. In addition to the fact that treatment bath management is quite complicated in the case of surface conditioners, a surface-conditioning step also requires installation of the corresponding treatment facilities and expansion of the space devoted to treatment. As a result, strong demand has recently appeared for the development of a conversion accelerator that is able to form high-quality conversion coatings on metal surfaces even without the implementation of a surface-conditioning step.